The invention relates to synchronization in telecommunication systems.
Digital communication networks often require a common timing reference to operate accurately. That is, the clocks in one node of the network should operate at the same speed as the clocks in other nodes of the network. Failure to provide synchronized clocks will lead to Jitter and Wander, which in turn can lead to such problems as transmission errors and buffer under/overflow. A network cannot maintain low error rates under such conditions, and ultimately may require some degree of unavailability to rectify the situation.
To provide for a common timing reference, digital communication networks include synchronization networks, whose job it is to ensure that a common timing reference is used throughout the network. One such synchronization network is described in European Telecommunication Standards Institute (ETSI) document European Guide (EG) 201 793 v1.1.1 (2000-10), entitled xe2x80x9cTransmission and Multiplexing (TM); Synchronization Network Engineeringxe2x80x9d, which is hereby incorporated herein by reference in its entirety. This document describes the various elements that make up a synchronization network, and the principles of operation by which such a network distributes accurate timing information from so-called Primary Reference Clocks (PRCs) to the clocks located in other pieces of equipment throughout the network. PRCs are the highest quality clocks in a network, and are usually based on a free-running Caesium Beam oscillator giving a very accurate clock performance.
FIG. 1 is a block diagram of an exemplary digital communication network 100 that includes a synchronization network. For purposes of illustration, the exemplary network 100 is a telecommunications network, and therefore includes, at network nodes, equipment that is well-known in the art. In the figure, transport links are indicated by solid lines, and synchronization reference links are shown by dashed lines that include an arrow at one end to indicate the source and recipient of the reference clock signal. Where a node has the possibility of receiving a reference clock from more than one source, primary reference links (i.e., those synchronization links that are preferred to be used for supplying a reference clock from one node to another) are denoted by the number xe2x80x9c1xe2x80x9d next to the dashed line indicating the link. Secondary reference links (i.e., those synchronization links that are used when the primary synchronization link is unavailable) are denoted by the number xe2x80x9c2xe2x80x9d next to the dashed line indicating the link.
The exemplary network 100 utilizes the Synchronous Digital Hierarchy (SDH), which is a standard technology for synchronous data transmission on optical media. It is the international equivalent of the Synchronous Optical Network (SONET). To facilitate the following discussion, the various nodes of the network are given reference characters A, B, C, D, E, F, G, H, I, L, M, and N.
In a fully synchronized network, all sources should be ultimately traceable to a PRC. In the exemplary network, this is the PRC A. The PRC A supplies its high quality clocking signal (xe2x80x9cclockxe2x80x9d) to Stand Alone Synchronization Equipment (SASE) B. A SASE is a piece of synchronization equipment that contains a Synchronization Supply Unit (SSU), which is a high quality slave clock. The SASE B distributes its clock signal to a Digital Exchange C (which, in alternative embodiments, could be a Telephone Exchange) and also to an SDH multiplexer (MUX) D.
The SDH MUX D distributes its clock signal to an SDH Digital Cross Connect unit (SDH DXC) E, which in turn distributes its clock signal to an SDH Add Drop Multiplexer (ADM) F. The clock supplied by the SDH ADM F is then supplied to each of two more SDH ADMs G and I. The reference link between the SDH ADM F and the SDH ADM G is a primary link.
Rather than using the supplied clock signal itself, the SDH ADM I operates in a xe2x80x9cbypassxe2x80x9d mode (commonly named xe2x80x9cNON-SETS lockedxe2x80x9d, where xe2x80x9cSETSxe2x80x9d stands for xe2x80x9cSynchronous Equipment Timing Sourcexe2x80x9d), whereby the synchronization clock is merely forwarded directly to the SASE L. This is common when, for example, the ADM and SASE are implemented in the same building. Essentially, the SASE L is the real recipient of the synchronization clock supplied by SDH ADM F, and this clock is treated as a secondary link. In the exemplary embodiment, the SASE L""s primary link is supplied (through the SDH ADM I operating in xe2x80x9cbypassxe2x80x9d mode) by an SDH ADM H.
Despite its bypass function, the SDH ADM I does require a synchronization clock, and in the exemplary embodiment this is supplied by the SASE L.
The SDH ADM I supplies its synchronization clock to the SDH ADM H, and this is treated as a secondary link. The SDH ADM H""s primary link is supplied by the SDH ADM G. To permit reconfigurability, the SDH ADM H is also coupled to supply a synchronization clock to the SDH ADM G, and this is treated as a secondary link by the SDH ADM G.
In accordance with the exemplary embodiment, the SDH ADM H also supplies a synchronization clock to a digital switch M, which also receives a synchronization clock from the digital switch N. The remainder of the exemplary network is not shown, since this is not important to understanding the invention.
It is very important that the synchronization network be planned in such a way so as to avoid the occurrence of timing loops, both during normal operation as well as when a malfunction prevents one or more nodes from supplying their reference clocks to their planned recipient nodes. A timing loop is created when a clock is directly or indirectly synchronized to itself. In a timing loop situation, all the clocks belonging to the loop can show a large frequency offset compared to nominal frequency and are likely to be isolated from the rest of the synchronization network. To avoid timing loops, elements in a ring should be provided with means that enable the possible generation of timing loops to be discovered. Such elements are usually connected such that they each have at least two synchronization sources, so that when one source is discovered to cause a timing loop, there is at least the possibility of avoiding it by selecting one of the alternative sources. For example, suppose that the reference link between nodes F and G is cut. In this situation, the SDH ADM G will respond by looking to node H to supply the necessary reference clock. However, under normal circumstances, node H expects to receive its reference clock from node G. It is apparent that a timing loop will occur here unless node H also responds to the break between nodes F and G by looking to another source for its reference clock. It is important that the clock supplied by this alternative source also not ultimately be derived from the clock at node G or from the clock at node H in order to avoid a timing loop.
In SDH networks, the use of Synchronous Status Messages (SSMs) provides some help with avoiding timing loops. An SSM is a signal that is passed over a synchronization interface to indicate the Quality-Level of the clock that the interface is ultimately traceable to; that is, the grade-of-clock to which it is synchronized directly or indirectly via a chain of network element clocks (the xe2x80x9csynchronization trailxe2x80x9d), however long this chain of clocks is. In a fully synchronized network, all sources should ultimately be traceable to a PRC, and there is a predefined code to indicate this. Another code, xe2x80x9cDo Not Use for Synchronizationxe2x80x9d, is used to prevent timing loops and is transmitted in the opposite direction on interfaces used to synchronize an equipment""s clock.
Although the SSM algorithm is a good concept in some applications like SDH or SONET rings, it is unable to guarantee that all timing loops will be prevented, because it only provides information about the quality of the traceable synchronization reference source, but not information about the actual physical source. See, for example, chapter 4.13 of ETS 300 417-6-1, xe2x80x9cGeneric requirements of transport functionality of equipment: Synchronization layer functionxe2x80x9d, which is hereby incorporated herein by reference in its entirety. Another drawback of the SSM algorithm is that it is often not supported by SASE or by Network Elements other than SDH/SONET Network elements (i.e., it can only be used between SDH/SONET Network Elements).
It is noted that timing loops can cause severe disturbances in the traffic network, but that the effect of these disturbances ver seldom gives a readily discernable indication of where the failure in the synchronization network occurred. It is therefore important to provide effective ways for managing synchronization networks so that, when failures occur in the network, it can be determined how to rearrange the network to maintain an acceptable quality of synchronization without creating timing loops.
Today, the management of synchronization networks is distributed among several platforms. The reason for this is that a synchronization network very often consists of different types of equipment that can be either dedicated to synchronization (e.g., SASE), or to both synchronization and traffic (e.g., and SDH multiplexer or a digital switch). As a result, several management systems (e.g., one for the SASE network, one for the SDH equipment, one for the switching network, etc.) have to be maintained in parallel. This situation is illustrated in FIG. 1, in which a first Switching Network Management Network 101 manages the Digital Exchange at node C; a SASE Management Network 103 manages the SASE at nodes B and L; a first SDH Management Network 105 manages the SDH Mux at node D, the SDH DXC at node E, and the SDH ADMs at nodes F and I; a second Switching Network Management Network 107 manages the Digital Switches at nodes M and N; and a second SDH Management Network 109 manages the SDH ADMs at nodes G and H. This situation is not unrealistic, since the xe2x80x9csamexe2x80x9d type of equipment (e.g., an SDH ADM) may be manufactured by different vendors who design their equipment using incompatible equipment management strategies. Such equipment may, nonetheless, be connected together in a single network to achieve diverse goals, such as supporting mobile network functions on the one hand, and hard-wired telephone functions on the other.
This problem of distributed management will only get worse in the future as new types of equipment (e.g., Internet Protocol (IP) routers) increase their synchronization function (due to new network application of this equipment), which will in turn require that this new equipment be managed from a synchronization perspective.
The above describes the best case situation of the conventional synchronization management approach. In practice, the network management of synchronization networks more often does not exist, or is incomplete. Because of this, conventional synchronization networks not only have to be well planned, but also need to be continuously maintained by maintenance personnel who may need to be physically present at the numerous sites. In most cases, this physical presence at each site is impossible due to geographical distribution of the synchronization network.
For the above reasons, it is very difficult for an operator of a conventional system to have full control and visibility of the synchronization network. Better synchronization management techniques and systems are therefore desired.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in methods and apparatuses that operate a synchronization network that includes a plurality of nodes and logic that distributes reference clocks to each of the nodes. Operation of the synchronization network includes, at each of the nodes, storing a table that represents a most recent status of the synchronization network. A change in synchronization status at a first node in the synchronization network is detected, and at the first node, the table is updated to represent a first updated status of the synchronization network. A synchronization network management protocol is then used to distribute the first updated status to other nodes in the synchronization network. In this way, each node in the synchronization network can have complete information about the most recent status of the synchronization network, thereby facilitating management of the network.
In another aspect of the invention, one or more other nodes may change their own status in response to receipt of information identifying a change in status at another node. By similarly updating their own tables and using the synchronization network management protocol to distribute the updated status to other nodes in the synchronization network, an iterative process is used that results in each node in the synchronization network having the most recent status about the synchronization network.
In some embodiments, the synchronization network may include an integrated synchronization network management node that also receives the synchronization network status information, and uses this to control all of the nodes in the synchronization network.